Process for cleaning aluminum



United States Patent Ofiice 3,307,799 Patented Feb. 6, 1968 3,367,709 PROCESS FOR CLEANlNG ALUMINUM Frederick J. Beyerle, Huntsville, Ala, assignor to the United States of America as represented by the Secretary of the Army N Drawing. Filed Oct. 8, 1963, Ser. No. 314,816 4 Claims. (Cl. 134-3) ABEiTRACT 0F THE DISQLOSURE A method for treating aluminum and aluminum alloys using a deoxidizer composition which is an aqueous solution of sulfuric acid, hydrofluoric acid, and an alkali metal dichromate. The aluminum or-aluminum alloy thus treated is in condition for resistance welding.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to a process for treating aluminum and aluminum alloys and to compositions useful in the treating process. More particularly, the invention relates to a process for cleaning aluminum and almuinum alloys prior to contact resistant welding operations and compositions useful in the cleaning process. As used herein, the terminology aluminum and aluminum alloys refers to the pure metal, commercial aluminum having only traces of impurities, and alloys of aluminum in which aluminum is a major constituent.

The problem of cleaning the surfaces of objects made from aluminum and aluminum alloys exists in practically all areas in which work with such materials is performed. However, where aluminum and its alloys are to undergo electric welding, the problem is particularly acute since it is impossible to achieve strong, uniform welds unless the welded surface is clean.

Many methods involving the use of strong caustic soda solutions and/or strong acid solutions will remove the oxide, A1 0 from the surfaces. These solutions, however, attack the aluminum itself and cause severe etching of the surface which increases the average electric resistance. Moreover, the strong solutions seem to leave other products on the treated surfaces which further increase the electrical resistance and thus, impedes welding. The electrical resistance of an aluminum object or aluminum alloy object that is to be electrically Welded should be less than 100 micro-ohms and, preferably, 50 microohms or less.

A method has been developed whereby the surfaces of objects of aluminum and aluminum alloys can be cleaned and the electrical resistance thereby reduced below 50 micro-ohms. Moreover, the method does not severely mar the surface. The method utilizes novel deoxidizing compositions which also form part of the invention.

In accordance with the foregoing, it is an object of the present invention to provide a method for treating the surface of aluminum and aluminum alloys.

Another object of the invention is to provide a method for cleaning the surface of aluminum and aluminum alloys.

A further object of the invention is to provide a method for cleaning aluminum oxide from the surface of aluminum and aluminum alloys.

A still further object of the invention is to provide a method for treating the surface of aluminum and alumi- 2111111 alloys to reduce the electrical resistance of the surace.

The manner in which these and other objects may be accomplished will become apparent from the following detailed description.

According to the present invention, the surfaces of an object of aluminum and aluminum alloy is first degreased using conventional techniques. After treating the surfaces with the degreasing solvent, the object is transferred immediately to an inhibited alkaline cleaning solution and subsequently rinsed with hot Water and then cold water. The surface having been thus treated is free of superficial dirt, grease, and scale. The object is next treated with a deoxidizer composition consisting essentially of an aqueous solution of sulfuric acid, hydrofluoric acid, and an alkali metal dichrornate. A cold water rinse is used to remove the deoxidizer composition from the surfaces of the objects and the object is thereafter dried, usually by placing it in an oven or subjecting it to blasts of hot air. The object thus treated is in condition for resistance welding or any other operation such as painting.

For degreasing the surfaces of the object the vapors of trichlorethylene, ethylene dichloride, or other organic solvent are satisfactory. It is acceptable to accomplishing the degreasing step by immersing the object in the liquid solvent although treatment with vapors at higher temperatures F. to 200 F.) expedites the removal of oil and grease films. The time required for this operation will depend on such variables as temperature of the solvent and amount of film present but will usually be within the range of five to twenty minutes.

Any of the alkaline cleaners normally used in the cleaning of aluminum and its alloys is acceptable for the alkaline cleaning solution used in preparing the surfaces for treatment with the deoxidizing solution. It is desirable to avoid the use of cleaners having soaps, aluminates, borates, silicates, and phosphates whenever possible. Solutions of active sodium oxide using about 20 grams of the oxide per liter of water are excellent. A commercial cleaner sold under the name Wyandotte Altrex is an acceptable cleaner when used in an amount of about 40 to 45 grams per liter of water although it does contain silicates. The alkaline bath will ordinarily be maintained at a temperature of about F. to about 190 F. and preferably at about F.

After the object is removed from the alkaline solution it is thoroughly rinsed with hot water, normally at a temperature of about 100 F. to about 140 F. and preferably at about 120 F. The time required for the hot water rinse will usually be from two to five minutes. The object is subsequently subjected to a cold water rinse, the temperature of this normally being within the range of 65 F. to 75 F. Since this is inclusive of room temperature (about 68 F.), ordinary tap water is generally suitable for the cold water rinse.

The object is now ready for subjecting it to the action of the deoxidizing solution. This solution is prepared by thoroughly mixing sulfuric acid, hydrofluoric acid, and alkali metal dichromate, and water. The solution ordinarily will contain about 8% to about 12% sulfuric acid, about 0.75% to about 1.25% hydrofluoric acid, and 5% to 7% of an alkali metal dichromate. The preferred deoxidizing solution of the invention is an aqueous solution of about 10% sulfuric acid, about 1.0% hydrofluoric acid, and about 6% sodium dichromate. The percentages given refer to the percent by weight based on the total weight of the solution including the water. The deoxidizing composition is maintained at a temperature of about 65 F. to about 75 F. when treating the surfaces of the object being cleaned. Ordinary room temperature (about 68 F.) is completely satisfactory. The surfaces of the object are usually kept in contact with the solution for 3 to 30 minutes depending upon type of alloy and amount of oxide to be removed, after which the object is thoroughly rinsed with cold water, that is, water at a temperature within the range of 65 F. to 75 F. Aluminum alloys of the 2000 and 7000 series generally require the greater time of contact with the deoxidizer solution, I claim: that is, 15 to 30 minutes. It is essential that all of the l. The method of treating the surface of aluminum deoxidizer composition be removed from the surface of and aluminum alloys prior to welding, said aluminum the treated object. The object is then dried in an oven at and aluminum alloys comprising at least 50% aluminum, about 150 F. or dried with hot air. 5 which method comprises subjecting said surface to the The object treated in the manner indicated above exaction of a deoxidizing composition for a period of about 'hibits a surface electrical resistance of less than 50 micro- 3 to about 30 minutes, said composition consisting essenohms. Moreover, the low level of electrical resistance is tially of an aqueous solution of about 8% to about 12% long lasting. Samples of 6061 aluminum alloy exhibited sulfuric acid, about 0.75% to about 1.25% hydrofluoric a surface resistance of 25 micro-ohms after treatment acid, about 5% to about 7% of an alkali metal dichroaccording to the present invention. At the end of 60 mate, and the remainder water, wherein all percentages hours, the resistance had risen only to about 33 microrefer to percent by weight of the total weight of compoohms. Similarly, samples of 5086 aluminum alloy had a sition, and thereafter rinsing said surface with water. surface resistance of about 23 micro-ohms when treated 2. The method according to claim 1 wherein said deas indicated above and the resistance increased to only oxidizing composition is maintained at a temperature 35 micro-ohms at the end of 60 hours. within the range of 65 F. to 75 F.

The specific process used in cleaning the 6061 and 5086 3. The method of treating the surface of aluminum aluminum alloy samples discussed above is outlined in and aluminum alloys to reduce the electrical resistance Table I below. thereof prior to welding, said aluminum and aluminum TABLE I Immersion Tank No. Solution Concentration Solution Temperature Time (minutes) Trichlorethyleneh 5-20 Wyandotte Altrex. 10 Hot Water Rinse 3 Cold Water Rinse H 1 Deoxidizerfi H2804, HF, Na C-r O ms o 10% (by wt.) ;HF,1.0% (bywt.); Na cr o 3 6%( by wt.); H20, remainder of solution Cold Water Rinse i. o 3 Drying Operation A 150 F. in oven 1 Technical Grade. ZSJulSire Acid, Technical Grade, 95-98%; Specific Gravity 1.84; Hydroi'luoric acid, Reagent Grade, 48%; Sodium Dichromate, CP Crystals.

I1 1 ry.

If demineralized water is used in the process, especially alloys comprising at least 50% aluminum, which method in preparing the alkaline solution and the deoxidizer comcomprises first cleaning superficial dirt and grease from position, better results may be obtained. However, this said surface and thereafter subjecting said surface to the is not essential to achieving completely satisfactory reaction of a deoxidizing composition for a period of about sults. 3 to about 30 minutes, said deoxidizing composition con- The solutions used in the present invention attack steel, sisting essentially of an aqueous solution of about 10% copper, brass, bronze, and magnesium. Therefore, desulfuric acid, about 1.0% hydrofluoric acid, about 6% vices incorporating such materials are generally not sodium dichromate, and the remainder water, wherein all treated in the assembled stage but routed for treatment percentages refer to percent by weight of the total weight of the aluminum and aluminum alloy portions only. of composition, and subsequently rinsing said surface Once the sequence of operations has started with any with water, the temperature of said deoxidizing compoobject, it should not be interrupted until completed, exsition being maintained in the range of 65 F. to 75 F. cept that reimmersion in the hot alkaline solution is nec- 4. The method according to claim 3 wherein said suressary if water breaks appear on the surface upon reface is dried after rinsing. moval from the hot water rinse. A minimum of time out Ct d of solution is desirable with parts during the sequence; References l e however, upon removing the parts from the various solu- UNITED STATES PATENTS tions and rinses, time should be allowed for the object 217L041 9/1939 Urban 7 X to drain to prevent contamination of the succeeding baths. 428,749 10/1947 De Long 156 22 X Af e an Ob]Ct has been cleaned, I". should be handled 2 20 2 5 12/1952 Hesch 156 21 With. clean cotton gloves until all lfiSlStfil'lCfi Welding has 2 705 500 4 1955 D 134 3 been completed. If an ob ect is not welded within about 2,719,796 V1955 Kappes et a1 134 3 X seventy-two hours, it should be protected from eontam- 2,829,091 4/1958 Misse1 matlon- L 2,867,514 1/1959 Newhard et a1. 134-3 X For best results, the operating cond1t1ons and solutions 3,197,340 7/1965 Beninger 134 3 X specified in Table I should be adhered to as closely as possible since Table I sets forth the preferred embodiment FOREIGN PATENTS of the invention including both the method and the de- 541,570 5/1957 Canada. oxidizing composition. 574,982 4/1959 Canada.

No undue limitation should be attributed to the present invention from the above description except as re- MORRIS WOLK Pflmmy Exammer fiected in the appended claims. I. ZATARGA, Assistant Examiner. 

